An imaging technique using an ultrasound wave employs an electroacoustic conversion element (transducer) to convert an electric signal into an ultrasound wave, to irradiate an object with the ultrasound wave, and the electroacoustic conversion element further receives a reflected wave (an echo) which is reflected from the object, so as to convert the reflected wave into an electric signal, thereby displaying on a monitor, image data that is generated based on the electric signal and time-series data. The ultrasound wave passes through the object and a part thereof is reflected on a boundary between different acoustic impedances, and an echo signal having strength depending on a difference of the impedances is generated. Therefore, a display is created assuming the boundary plane as a tomographic image of the object. This kind of imaging technique as described above is widely employed in a nondestructive inspection of a structural object, or as a diagnostic apparatus for performing a minimally invasive imaging to take a tomographic image of a living body.
Along with propagating through the object, the irradiated ultrasound wave may have acoustic waveform distortion. This is because there is a phenomenon caused by acoustic nonlinearity that a part with a high sound pressure in the transmitted acoustic waveform progresses fast, whereas a part with a low sound pressure progresses slowly. Since the longer the acoustic wave propagates, the more this phenomenon is accumulated, and therefore, this intensifies the waveform distortion.
An imaging method for achieving a higher image quality, utilizing this acoustic nonlinearity, is provided in an ultrasonic diagnostic apparatus. When the inner side of a living body is irradiated with the ultrasound wave, waveform distortion is generated in the process of propagation, and a nonlinear component made up of harmonics is generated, in addition to a fundamental frequency component of the irradiated acoustic wave. This nonlinear component is generated in proportion to approximately the square of the amplitude of fundamental wave sound pressure. Therefore, it is possible to create an image with a higher contrast, compared to a normal imaging method according to the fundamental wave, and an image with a high resolution may be obtained. This type of imaging for taking an image of the nonlinear component in a living body tissue is referred to as THI (Tissue harmonic imaging).
In the imaging method according to the THI, the reflection echo strength being generated by the nonlinear component is smaller than the echo strength of the fundamental wave component. Therefore, in order to perform imaging by using only the nonlinear component, it is necessary to separate the nonlinear component from the fundamental wave component to extract the nonlinear component. Conventionally, as a method for extracting the nonlinear component, there are known a method that uses a filter to separate the nonlinear component (e.g., Patent Document 1), PI (Pulse inversion) method (e.g., Patent Document 2), and an amplitude modulation method (e.g., Patent Document 3). Those methods will be explained briefly in the following.
Firstly, a brief explanation will be made as to the method that uses a filter to separate the nonlinear component. When an ultrasound wave with the center frequency f0 is transmitted, the echo signal obtained from the living body contains in a mixed manner, a fundamental wave component (linear component) generated around the center frequency f0 being the same as the transmission frequency, a second higher harmonic component (nonlinear component) generated around the frequency 2f0 according to acoustic nonlinearity, and the like. The nonlinear component is generated in a higher frequency region relative to the linear component, and therefore, by filtering the high frequency region, it is possible to extract the nonlinear component.
The PI (Pulse inversion) method transmits two ultrasound pulses, one sound pressure waveform being reversed from the other; positive and negative, to an identical portion of the living body, and adds reflection echoes thereof together. Since the fundamental wave component behaves linearly, when transmission pulses being inversed with each other are transmitted, fundamental wave components of the reflection echoes are also inverted with each other, and they are canceled out by adding together. On the other hand, the nonlinear components (second higher harmonic components) are distorted in a different manner between on the positive side and on the negative side of the sound pressure. Therefore, even though transmission pulses being inverted with each other are transmitted, they do not become waveforms being inverted, and they are not canceled out by adding together. Therefore, if the reflection echoes of the inverted transmission pulses are added together, this may result in that only the nonlinear component remains.
As described in the Patent Document 3, in the amplitude modulation method, transmission of ultrasound wave is performed twice similar to the PI method, but as for the second transmission pulse, sound pressure level (amplitude) is reduced relative to the first transmission pulse, without inverting the sound pressure waveform. By way of example, the amplitude is reduced into half. A nonlinear component (second higher harmonic component) is generated in proportion to the square of the sound pressure of a fundamental wave component. Therefore, the sound pressure of the nonlinear component in the echo signal of the second transmission becomes a quarter relative to the echo signal of the first transmission. Thus, the echo signal of the second transmission is doubled and subtracted from the echo signal of the first transmission, thereby canceling out the fundamental wave components, resulting in remaining of only the nonlinear components.